1. Field of the Invention
This invention relates to any two-stage gas-cleaning electrostatic precipitator for removing particles from a gas. More particularly, it relates to an improved collecting means for precipitating ionized particles.
A two-stage gas-cleaning electrostatic precipitator usually consists of an ionizing stage, a collecting stage and a means for causing gas needing to be cleaned first to pass through the ionizing stage and then through the collecting stage. The particles to be removed from the gas are ionized or given a charge when the gas passes through the ionizing stage. The charged particles pass into the collecting stage where they are precipitated. The precipitation occurs because there is a voltage gradient in the spaces between the plates of the collecting stage which acts on the charged particles, moving them to the plates and thus out of the gas stream.
2. Description of the Prior Art
The Cottrell-type electrostatic precipitator has been used since about 1905 to remove particles from smoke stack gases. In a Cottrell-type electrostatic precipitator the particles are both charged and precipitated in the same corona region. As a result, this type of electrostatic precipitator ("ESP") is called a single-stage ESP. A single-stage ESP, however, has several drawbacks. It cannot be used for cleaning ventilating air because it generates excessive ozone. Also a single-stage ESP is usually large and requires a high voltage, typically 40 KV to 100 KV.
My U.S. Pat. No. 2,129,783, issued July 26, 1938, describes a much more compact and lower voltage ESP. This ESP only uses corona to charge the particles which are then removed in a corona-free region whereas the Cottrell-type precipitator uses corona both to charge and precipitate the particles. As a result, this type of ESP is called a two-stage ESP. Only a very small amount of ozone is generated in a two-stage ESP and therefore it can be used for cleaning ventilating air. In the two-stage ESP described in the above patent, the plates of the particle collecting section are spaced about 0.25 inches apart. This compares to a spacing of 4.0 inches between high and low voltage electrodes in the Cottrell-type ESP.
To a first approximation, a given cleaning capacity measured in cubic feet per minute (CFM.) requires a given particle or dust collecting area. Reducing the spacing between electrodes reduces the size of the ESP, but it also reduces the dust holding capacity. Consequently, a two-stage ESP, typically, has been used only for relatively low particle loadings except for the case of oil droplets where the collected liquid can continuously drain from the collecting electrodes. Thus, there is a need for a two-stage ESP capable of handling high particle loadings.
Typically, two-stage precipitators are operated with a gas velocity at the face of the collecting section of 300 ft./min. to 400 ft./min. At this gas velocity and with high particle loadings the collecting section will require frequent cleaning. Normally this is done by shutting down the precipitator and removing the collecting section for cleaning with water. A preferable cleaning mechanism would be one which can function effectively during the operation of the precipitator without shutting it down.
Two-stage precipitators can also cause a space charge problem in rooms supplied with imperfectly cleaned air. This problem is described in the paper entitled "Electrically Charged Dust in Rooms" by Gaylord W. Penney and George W. Hewitt, Trans. of Amer. Inst. of Electrical Engrs., 1949. Space charge can be summarized as follows: (1) all of the particles to be precipated are given a charge of one sign by passing them through a corona discharge; (2) if these charged particles are not removed by the collecting stage, a space charge is created in the room supplied by this partially cleaned air; (3) this space charge drives any other charged particles toward the room's walls making them very dirty.
My U.S. Pat. No. 2,948,353, issued Aug. 6, 1960, described a two-stage ESP with much closer spacing between the high and low voltage electrodes (between 0.02 inches and 0.08 inches). At this close spacing, it is expected that short circuits between electrodes will occur frequently and so the precipitator is designed to operate with some electrodes short circuited. This is accomplished by two different methods. In the first method a number of electrodes are operated in series using semi-conducting spacers and a current limiting series resistance. In the second method insulating spacers are used between the electrodes with every other electrode connected together and grounded. The alternate or high voltage electrodes located between the connected and grounded electrodes are each connected to a high voltage power source through a current limiting series resistance.
The 2,948,353 patent, however, has not been used commercially because suitable high resistance, low cost materials, with the required uniformity of resistance, have not been available for use as either the spacers or the current limiting series resistance. Also, even if suitable resistance materials were available, the particle holding capacity of the invention described by the 2,948,353 patent is very limited.